Hydraulic power transmissions in which a hydrostatic transmission (HST) is disposed in a housing and in which an oil pump is formed in the housing are well known in the art as disclosed in U.S. Pat. Nos. 4,914,907; 4,932,209; 4,986,073 and 4,903,545.
All the above-mentioned hydraulic power transmissions have been constructed to provide a counter-measure for leakage of operating oil from the HST so that the hydrostatic transmission takes in oil leaked into the housing for use in the closed fluid circuit within a central section of the HST.
The Applicant of the present invention has previously disclosed (in the U.S. Pat. No. 5,031,403) the provision of a charge pump for positively supplying operating oil to the hydrostatic transmission, disposed together therewith in a housing.
In the hydraulic power transmission in U.S. Pat. No. 5,031,403 when the oil in the housing is pressurized enough to be taken out the housing for actuating an actuator for lifting a working machine, a pump separate from the charge pump must also be disposed in the housing. Therefore, two pumps are required, increasing the expense to produce and the space required to contain the two pumps, resulting in a hydraulic power transmission that is large in sized as a whole.
Also in this previous transmission, an oil filter for filtering the oil in the housing when it is taken in by the charge pump is disposed together therewith in the housing. In order to periodically change the filter to avoid it being clogged, the housing needs to be dismantled, thereby increasing maintenance costs.